Back to EveryPatent.com
| United States Patent |
5,246,779
|
|
Heimberg
, et al.
|
September 21, 1993
|
Microfine propylene polymer powders and process for their preparation
Abstract
The present invention is directed to improved microfine propylene polymer
powders having reduced fines which are comprised of spherical or
substantially spherical particles and have a number average particle size
from about 25 to about 60. A process for preparing the improved powders is
also provided. The process involves dispersing a molten propylene
homopolymer or copolymer which has been visbroken in a liquid medium in
the presence of a nonionic surfactant, cooling the dispersion below the
melt point of the propylene polymer and recovering the powder.
| Inventors:
|
Heimberg; Manfred (Cincinnati, OH);
Ondrus; Daniel J. (Farmington Hills, MI)
|
| Assignee:
|
Quantum Chemical Corporation (New York, NY)
|
| Appl. No.:
|
927750 |
| Filed:
|
August 10, 1992 |
| Current U.S. Class: |
428/402; 525/333.8; 525/387; 525/938; 528/491; 528/494 |
| Intern'l Class: |
C08F 006/24; C08F 008/50 |
| Field of Search: |
428/402
528/491,494,499
525/333.8,387,938
|
References Cited
U.S. Patent Documents
| 3144436 | Aug., 1964 | Greene et al. | 260/93.
|
| 3422049 | Jan., 1969 | McClain | 260/29.
|
| 3432483 | Mar., 1969 | Peoples et al. | 260/87.
|
| 3563970 | Feb., 1971 | Leicht et al. | 528/494.
|
| 3746681 | Jul., 1973 | McClain | 260/29.
|
| 3940739 | Feb., 1976 | Quimet | 340/164.
|
| 3970719 | Jul., 1976 | Edmonds | 260/878.
|
| 4039632 | Aug., 1977 | Edmonds | 260/878.
|
| 4061694 | Dec., 1977 | Castagna | 260/878.
|
| 4504653 | Mar., 1985 | Kuwabara et al. | 528/494.
|
Other References
Encyclopedia of Chemical Technology, vol. 16, Third Edition, pp. 453-465,
John Wiley & Sons (New York).
Encyclopedia of Polymer Science & Engineering, vol. 13, pp. 464-479, John
Wiley & Sons (New York).
Product Finishing, pp. 22-27 (9/1990).
|
Primary Examiner: Teskin; Fred
Attorney, Agent or Firm: Tremain; Kenneth D., Baracka; Gerald A.
Claims
We claim:
1. In a process for producing propylene polymer powders comprising the
steps of:
(1) heating a propylene polymer to above the melt point of the polymer with
a nonionic surfactant which is a block copolymer of ethylene oxide and
propylene oxide and a polar liquid medium which is not a solvent for the
propylene polymer; said nonionic surfactant present in an amount from 4 to
50 percent, based on the weight of the propylene polymer, and the weight
ratio of said polar liquid medium to propylene polymer ranging from 0.5:1
to 10:1;
(2) dispersing the mixture to produce droplets of the desired size;
(3) cooling the dispersion to below the melt point of the propylene
polymer; and
(4) recovering the propylene polymer powder; to obtain powders having
reduced fines and improved particle size distribution; the improvement
wherein the propylene polymer is a visbroken propylene polymer having a
melt flow rate greater than 1.
2. The process of claim 1 wherein the propylene polymer is a homopolymer of
propylene or a copolymer of propylene with up to 25 weight percent
ethylene.
3. The process of claim 2 wherein the propylene polymer has a crystallinity
content from 40 to 75 percent.
4. The process of claim 2 wherein the melt flow rate of the visbroken
propylene polymer is from 2 up to about 100.
5. The process of claim 4 wherein the propylene polymer is visbroken by
contacting with an organic peroxide at an elevated temperature.
6. The process of claim 4 wherein the visbroken propylene polymer is
polypropylene having a melt flow rate from 2 to 40.
7. The process of claim 6 wherein the melt flow rate is 5 to 35.
8. The process of claim 4 wherein the visbroken propylene polymer is a
random copolymer of propylene and 1 to 10 weight percent ethylene having a
melt flow rate from 2 to 40.
9. The process of claim 8 wherein the melt flow rate is 5 to 35.
10. The process of claim 9 wherein the random copolymer contains 1 to 5
weight percent ethylene.
11. A visbroken propylene polymer microfine powder comprised of particles
which are spherical or substantially spherical in shape and having a
number average particle size from 25 to 60 microns and wherein particles
under 10 microns in size do not exceed 10 percent.
12. The microfine powder of claim 11 wherein the propylene polymer is a
homopolymer of polypropylene or a copolymer of polypropylene with up to 25
weight percent ethylene visbroken to a melt flow rate of 2 to 100.
13. The microfine powder of claim 12 wherein the propylene polymer has a
crystallinity content from 40 to 75 percent.
14. The microfine powder of claim 12 wherein the propylene polymer is
polypropylene having a melt flow rate from 2 to 40.
15. The microfine powder of claim 12 wherein the propylene polymer is a
random copolymer of propylene and 1 to 10 weight percent ethylene having a
melt flow rate from 2 to 40.
16. The microfine powder of claim 12 having a number average particle size
from 30 to 50 microns.
17. The microfine powder of claim 16 wherein particles under 10 microns in
size do not exceed 5 percent.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improved microfine propylene polymer
powders which are spherical or substantially spherical in shape. The
improved powders of the invention have reduced fines and a particle size
distribution which renders them useful for powder coatings and especially
for electrostatic spray coatings. The invention also relates to a process
for producing the improved propylene polymer powders.
2. Description of the Prior Art
Thermoplastic resin powders are widely used in industry and these uses are
well documented in the prior art. For example, powdered thermoplastic
resins in dry form have been used to coat articles by dip coating in
either a static or fluidized bed and by powder coating wherein the powder
is applied by flame or electrostatic spraying or dusting. Powders can also
be applied in dispersed form, by roller coating, spray coating, slush
coating, and dip coating substrates such as metal, paper, paperboard, and
the like. Powders are also widely employed for conventional powder molding
processes, e.g., rotational molding and rotational lining. Still other
applications for powders include use as paper pulp additives; mold release
agents; additives for waxes, paints, caulks, and polishes; binders for
non-woven fabrics; etc.
Besides the physical properties of the powder, which are dictated primarily
by the resin being used, the size and shape of the powder particles are
also important considerations in the selection of a powder for a
particular application. These latter properties are primarily a function
of the process by which the powders are prepared, which can include
mechanical grinding, solution processes or dispersion processes. Particle
size is determined using U.S. Standard Sieves or light scattering
techniques and, depending on the method used, will be reported in mesh
size or microns. The inverse relationship between the sieve size (mesh
number) and particle size (in microns) is well documented and conversion
tables are available. The shape of the particles is ascertained from
photomicrographs of the powders. Particle shape has a marked influence on
the bulk density and handling characteristics of the powder.
It is known that the presence of substantial amounts of "fines," i.e.,
particles having average diameters of 10 microns or less, can create
problems in electrostatic powder coating operations. Some of the most
common operational problems arising from the presence excess fines are
identified by D. S. Tyler, Product Finishing, September 1990, pp. 23-26.
Most significantly, the article points out that these fine particles have
so little mass that they tend to be carried away from the product to be
coated and into the reclaim system. The author cites studies which show
that virtually no particles having an average particle size of less than
10 microns are retained on the coated part and concludes that substantial
reduction or elimination of such particles is desirable.
Crystalline propylene polymers posses a desirable balance of toughness and
chemical and solvent resistance. Furthermore, their low cost, low specific
gravity, and low melt point make them useful for fusion coating processes.
The utility of propylene polymers has been somewhat limited, however, due
to the lack of availability of powders having suitable particle size
and/or particle size distribution.
Powders can be produced using dispersion techniques, such as those
described in U.S. Pat. Nos. 3,422,049 and 3,746,681. Such processes
produce particles which are spherical in shape and have a relatively
narrow size range, i.e., particle size distribution. These dispersion
procedures involve subjecting the molten resin in about 0.8 to 9 parts by
weight of water per part of resin to vigorous agitation in the presence of
from about 2 to 25 parts by weight per 100 parts of resin of a
water-soluble block copolymer of ethylene oxide and propylene oxide having
a molecular weight above about 3500 and containing at least about 50% by
weight of ethylene oxide so that a fine dispersion is produced. The
resulting dispersion is then cooled to below the softening temperature of
the resin and the powder recovered. A continuous dispersion process for
the preparation of finely divided polymer particles is disclosed in U.S.
Pat. No. 3,432,483. The process comprises the sequential steps of feeding
to the polymer, water and a water-soluble block copolymer of ethylene
oxide and propylene oxide surfactant into a dispersion zone; vigorously
agitating the mixture under elevated temperature and pressure to form a
dispersion of the molten polymer; withdrawing a portion of the dispersion
and cooling to a temperature below the melting point of said polymer to
form solid, finely divided polymer particles in the dispersion; reducing
the pressure of said cooled dispersion to atmospheric pressure; separating
the solid polymer particles from the surfactant solution phase and
washing; drying the washed polymer particles; and recovering dry, finely
divided powder.
While microfine powders of propylene polymers can be produced using the
above-described dispersion processes, the powders obtained typically have
a large number of particles which are 10 microns or less in size. The
presence of these fines not only limits the utility of the product but
also presents recovery problems. It would be highly desirable if microfine
powders of crystalline propylene polymers could be produced wherein the
amount of fines is significantly reduced. These and other advantages are
realized by the present invention which is described in more detail to
follow.
SUMMARY OF THE INVENTION
It has now quite unexpectedly been discovered that improved powders of
propylene polymers are produced when the polymer is visbroken before it is
subjected to the powder-forming dispersion operation. By using a visbroken
propylene polymer, it is possible to significantly alter the particle size
distribution of the resulting powder and, most advantageously, markedly
reduce the amount of fines in the powder. The powder particles produced by
the process are spherical or substantially spherical. While visbreaking
propylene polymers to increase melt flow rate and facilitate
processability is known, the ability to modify the characteristics of
powders produced therefrom is totally unexpected. When powders are
produced using dispersion procedures from visbroken and non-visbroken
propylene polymers having identical melt flow rates, the powder obtained
from the visbroken product has significantly reduced fines.
The process of the invention, to produce improved microfine propylene
polymer powders having reduced fines and improved particle size
distribution, comprises dispersing a visbroken propylene polymer and a
nonionic surfactant which is a block copolymer of ethylene oxide and
propylene oxide in a polar liquid medium which is not a solvent for the
propylene polymer, said nonionic surfactant present in an amount from 4 to
50 percent, based on the weight of the propylene polymer, and the weight
ratio of said polar liquid medium to propylene polymer ranging from 0.5:1
to 10:1, at a temperature above the melting point of the propylene polymer
to produce droplets of the desired size and then cooling the dispersion to
below the melting point of the propylene polymer and recovering the
propylene polymer powder. Propylene polymers which are visbroken and
utilized in accordance with the invention to produce the improved
microfine powders are homopolymers of propylene or copolymers of propylene
with up to 25 weight percent ethylene. Typically, the propylene polymers
have crystallinity contents of 40 to 75 percent. Propylene homopolymer and
random copolymers of propylene with 1 to 10 weight percent ethylene are
particularly advantageous.
In one embodiment of the invention, the propylene polymer is visbroken at
an elevated temperature in the presence of an organic peroxide. As a
result of the visbreaking operation, the melt flow rate of the propylene
polymer is increased by one unit or more. The visbroken propylene polymers
generally have a melt flow rates greater than 1 and less than 1000. Melt
flow rates after visbreaking are more preferably in the range of 2 to 100
and, even more preferably, 2 to 80. The resulting microfine powders are
comprised of particles which are spherical or substantially spherical in
shape and which have a number average particle size from about 25 to 60
microns. The powders contain less than about 10 percent fines, i.e.,
particles under 10 microns in size.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plot (% vs. particle size) obtained from the Malvern Particle
Size Analyzer illustrating the particle size distribution of powder
produced in accordance with the invention from a 20 MFR polypropylene
homopolymer obtained by visbreaking a 5 MFR reactor produced homopolymer
to 20 MFR prior to the powder forming operation.
FIG. 2 is a plot (% vs. particle size) obtained from the Malvern Particle
Size Analyzer illustrating the particle size distribution of a comparative
powder produced from a 20 MFR polypropylene homopolymer. The homopolymer
used for this comparison was not visbroken but rather reactor synthesized
to 20 MFR and the powder directly prepared therefrom.
FIG. 3 is a plot (% vs. particle size) obtained from the Malvern Particle
Size Analyzer which shows the particle size distribution of a powder
produced in accordance with the invention from a 9 MFR random
propylene-ethylene copolymer. A reactor-produced 2 MFR random copolymer
was visbroken to MFR 9 before forming the powder.
FIG. 4 is a plot (% vs. particle size) obtained from the Malvern Particle
Size Analyzer showing the particle size distribution of a comparative
powder produced from a reactor synthesized 10 MFR random copolymer of
propylene and ethylene which was not visbroken.
DETAILED DESCRIPTION OF THE INVENTION
To produce the improved powders of the present invention propylene polymers
having crystallinity contents as determined by x-ray diffraction from
about 40 percent up to about 75 percent are employed. Useful propylene
polymers can include homopolymers of propylene and copolymers of propylene
with up to about 25 weight percent ethylene.
Crystalline, isotactic propylene homopolymers are known and commercially
available. All of these polypropylene resins, which can vary in melt flow
rates and physical properties, are capable of being visbroken and can be
converted to microfine powders in accordance with the invention.
Crystallinity contents of the propylene homopolymers usually range from
about 50 percent up to about 70 percent and, more typically, from 55
percent to 65 percent.
Copolymers of propylene with ethylene, random and block, are also known and
can be used to produce useful powders in accordance with this invention.
Numerous polymerization procedures are described in the prior art for
their preparation of these random and block copolymers. Block copolymers,
for example, can be made in accordance with the processes of U.S. Pat.
Nos. 3,970,719 or 4,039,632.
The random copolymers will generally contain from 1 to 10 weight percent
ethylene and, more preferably, from 1 to 5 weight percent ethylene. The
block copolymers will generally contain from 5 to 25 weight percent
ethylene and, more preferably, from 5 to 20 weight percent ethylene. The
random and block propylene copolymers generally have crystallinity
contents of from 45 percent to 65 percent and, more typically, from 50 to
65 percent. Small amounts of other polymerizable monomers may be included
with the propylene and ethylene if desired.
Propylene homopolymers and copolymers of the above types are generally
discussed in Volume 16 of Kirk-Othmer's Encyclopedia of Chemical
Technology, 3rd Edition, pp. 453-467 and in Volume 13 of Encyclopedia of
Polymer Science and Engineering, 1988, pp. 464-530, the contents of which
are incorporated herein by reference.
To achieve the improved microfine powders of the invention, it is necessary
to employ a propylene polymer of one of the foregoing types which has been
visbroken. Visbreaking propylene polymers to increase their melt flow rate
and improve processability is known and numerous procedures are described
in the art, such as in U.S. Pat. Nos. 3,144,436, 3,940,739 and 4,061,694.
It is not known, however, that visbreaking is capable of influencing the
characteristics of propylene polymer powders produced using dispersion
procedures.
Visbreaking is the controlled degradation of propylene polymers and two
methods, thermal and chemical, are most commonly employed to effect this
modification. Both methods involve working the polymer at an elevated
temperature using a suitable mixer. Mixers which impart shear during the
mixing are preferentially used. Such mixers include single or twin-screw
extruders, Banbury mixers or the like. These operations are typically
carried out until the desired modification is achieved. In batch
operations, for example, the operation will be carried out for a period of
time sufficient to effect the desired melt flow rate change. In continuous
operations, such as where extruders are used to bring about the
visbreaking, residence time can be varied or multiple passes can be made
to increase melt flow rate change.
Thermal visbreaking or degradation of propylene polymers is generally
carried out at temperatures in excess of about 550.degree. F. and in the
absence of free radical initiators. For chemical visbreaking about 50 up
to about 2000 ppm free radical initiator, such as a peroxide,
hydroperoxide, azo or diazo compound is included with the polymer.
Chemical visbreaking is carried out at a temperature which is above the
melting point of the polymer and above the decomposition temperature of
the initiator, typically from about 350.degree. F. to 550.degree. F.
The visbroken propylene polymers from which the improved powders are
produced necessarily have melt flow rates, determined in accordance with
ASTM D1238, Condition L, greater than 1. Acceptable dispersions, i.e.,
dispersions having droplets of the requisite size to form fine powders,
cannot be produced with polymers having melt flow rates less than 1. It is
even more advantageous when producing powders using the dispersion process
that the visbroken propylene polymers have melt flow rates greater than
about 2. Aside from the foregoing limitation, melt flow rates of the
visbroken propylene polymers are governed by the particular application
for which the powder is intended. While melt flow rates of visbroken
propylene polymers may go as high as 1000 or above, equipment and process
limitations generally dictate that melt flow rates not exceed about 100 .
Accordingly, for most applications, the melt flow rate of the visbroken
propylene polymer, and the resulting powder formed therefrom, ranges from
2 to 100 and, more preferably, from 2 to 80. In a highly useful embodiment
of the invention the melt flow rate of the visbroken propylene polymer is
from 2 to about 40. In an even more advantageous embodiment, the melt flow
rate of the visbroken propylene homopolymer or copolymer used to produce
the powder is in the range 5 to 35.
The extent to which the original propylene polymer is visbroken is not
critical and primarily depends on the melt flow rate of the original
material, i.e., the propylene polymer as obtained from the polymerization
reactor, and the desired powder melt flow rate. All that is necessary for
the process of the invention to produce the improved powders is that the
melt flow rate of the visbroken propylene polymer be at least one unit
greater than the melt flow rate of the original reactor-produced propylene
polymer. In the usual practice of the invention, however, the increase in
melt flow rate will be greater than one unit.
While the various factors which contribute to the improvement in the
powders of the invention are not understood, it is known that the ability
to improve particle size distribution and significantly reduce the amount
of particles 10 microns or less in size is not solely a function of the
final melt flow rate of the propylene polymer. This is apparent from the
data showing that when visbroken and non-visbroken propylene polymers
having the same melt flow rates are dispersed, the powders produced have
significantly different powder characteristics. While both visbroken and
non-visbroken propylene polymers having melt flow rates above about 2 are
readily dispersible and form powders in the dispersion process, only
powders of visbroken polypropylene polymers have reduced fines and the
desired particle size distribution.
The microfine propylene polymer powders prepared in accordance with the
invention are comprised of particles which are spherical or substantially
spherical in shape and which have a number average particle size from
about 25 microns to about 60 microns and, more preferably, from about 30
microns to 50 microns. Particles under 10 microns in size will not exceed
10 percent. Powders having fines, i.e., particles under 10 microns,
contents of 5 or below are particularly useful. Melt flow rates of the
resulting improved microfine powders will not be significantly different
than that of the starting visbroken propylene polymer. Accordingly, powder
melt flow rates will generally not exceed 100 and more usually will range
from 2 to 80. The visbroken propylene polymer powders of the invention
having reduced fines are highly useful as electrostatic powder coatings.
The visbroken polypropylene polymer is converted to a microfine powder
using known dispersion procedures such as those of U.S. Pat. Nos.
3,422,049, 3,432,483 and 3,746,681, details of which are incorporated
herein by reference. For the powder-forming operation, the visbroken
propylene polymer is charged to the reactor with a polar liquid medium and
a nonionic surfactant and a dispersion is formed in accordance with
conventional dispersing procedures described in the art. The dispersing
apparatus may be any device capable of delivering sufficient shearing
action to the mixture at elevated temperature and pressure. Conventional
propeller stirrers designed to impart high shear which are commercially
available can be used for this purpose. The reactor may also be equipped
with baffles to assist in dispersion. Particle size and particle size
distribution will vary depending on the shearing action which, in turn, is
related to the stirrer design and rate of stirring. Agitation rates can
vary over wide limits but the speed of the stirrer will usually be
controlled so that the tip speed is between about 500 and 3500 ft/min and,
more commonly, 750 and 3000 ft/min. A higher tip speed is generally
required for batch operation, usually 2500-3000 ft/min. Tip speeds for
continuous procedures most generally range between 750 and 2500 ft/min.
The dispersion process is typically conducted in an autoclave since this
permits the process to be conducted at elevated temperature and pressure.
In the usual batch conduct of the process, all of the ingredients are
charged to the autoclave and the mixture is heated to a temperature above
the melting point of the propylene polymer. While the temperature will
vary depending on the specific polymer being used, it will typically range
from about 90.degree. C. to 250.degree. C. Since the fluidity of polymers
is temperature related, it may be desirable to carry out the process at
temperatures substantially above the melting point of the olefin copolymer
to facilitate dispersion formation. The temperature, however, should not
exceed the thermal degradation temperature of the polymer.
Stirring is commenced after the desired temperature is reached and
continued until a dispersion of the desired droplet size is produced. This
will vary depending on the particular propylene polymer being used, the
temperature, the amount and type of surfactant, and other process
variables but generally will range from about 5 minutes to about 2 hours.
Most generally, stirring is maintained for a period from 10 to 30 minutes.
A polar liquid medium which is not a solvent for the propylene polymer is
employed to form the dispersions. These polar mediums are hydroxylic
compounds and can include water, alcohols, polyols and mixtures thereof.
The weight ratio of the polar liquid medium to polymer ranges from about
0.8:1 to about 9:1 and, more preferably, from 1:1 to 5:1. It is
particularly advantageous to use water as the dispersing medium or to use
a liquid medium where water is the major component. The pressure of the
process is not critical so long as a liquid phase is maintained and can
range from about 1 up to about 217 atmospheres. The process can be
conducted at autogenous pressure or the pressure can be adjusted to exceed
the vapor pressure of the liquid medium at the operating temperature. Most
generally, with aqueous dispersions the pressure will range from about 5
to 120 atmospheres.
In order to obtain suitable dispersions, one or more dispersing agents are
necessarily employed. Useful dispersing agents are nonionic surfactants
which are block copolymers of ethylene oxide and propylene oxide.
Preferably, these nonionic surfactants are water- soluble block copolymers
of ethylene oxide and propylene oxide and have molecular weights greater
than about 3500. Most will contain a major portion by weight of ethylene
oxide and are obtained by polymerizing ethylene oxide onto preformed
polyoxypropylene segments. The amount of nonionic surfactant employed can
range from about 4 to 50 percent, based on the weight of the propylene
polymer. Most preferably, the nonionic surfactant is present from about 7
to 45 percent, based on the weight of the polymer.
Useful nonionic surface active agents of the above type are manufactured
and sold by BASF Corporation, Chemicals Division under the trademark
Pluronic. These products are obtained by polymerizing ethylene oxide on
the ends of a preformed polymeric base of polyoxypropylene. Both the
molecular weight of the polyoxypropylene base and the polyoxyethylene
segments can be varied to yield a wide variety of products. One such
compound found to be suitable for the practice of the process of this
invention is the product designated as F-98 wherein a polyoxypropylene of
average molecular weight of 2,700 is polymerized with ethylene oxide to
give a product of molecular weight averaging about 13,500. This product
contains 20 weight percent propylene oxide and 80 weight percent ethylene
oxide. Other effective Pluronic.RTM. surfactants include F-88 (M.W.
11,250, 20% propylene oxide, 80% ethylene oxide), F-108 (M.W. 16,250, 20%
propylene oxide, 80% ethylene oxide), and P-85 (M.W. 4,500, 50% propylene
oxide, 50% ethylene oxide). These compounds, all containing at least about
50 weight percent ethylene oxide and having molecular weights of at least
4,500, are highly effective as dispersing agents for the aforementioned
propylene polymers.
It is also possible to employ products sold under the trademark Tetronic
which are prepared by building propylene oxide block copolymer chains onto
an ethylenediamine nucleus and then polymerizing with ethylene oxide.
Tetronic.RTM. 707 and Tetronic.RTM. 908 are most effective for the present
purposes. Tetronic.RTM. 707 has a 30 weight percent polyoxypropylene
portion of 2,700 molecular weight polymerized with a 70 weight percent
oxyethylene portion to give an overall molecular weight of 12,000.
Tetronic.RTM. 908, on the other hand, has a 20 weight percent
polyoxypropylene portion of 2,900 molecular weight polymerized with an 80
weight percent oxyethylene portion to give an overall molecular weight of
27,000. In general, useful Tetronic.RTM. surfactants have molecular
weights above 10,000 and contain a major portion by weight of ethylene
oxide.
The powder-forming process may also be conducted in a continuous manner. If
continuous operation is desired, the ingredients are continuously
introduced to the system while removing the dispersion from another part
of the system. The ingredients may be separately charged or may be
combined for introduction to the autoclave.
The following examples illustrate the process of the invention and the
improved powders obtained therefrom more fully. As will be apparent to
those skilled in the art, numerous variations are possible and are within
the scope of the invention.
Melt flow rates referred to in the examples were measured in accordance
with ASTM D1238-89 at 190.degree. C. at 230.degree. C. with a Tinius Olsen
Extrusion Plastometer. The melt flow rate (MFR) is expressed in grams per
10 minutes.
An electrically heated two-liter Paar pressure autoclave equipped with
thermowell and thermocouple connected to a digital display was used to
prepare the dispersions. The autoclave was equipped with an agitator and a
Strahman valve to permit rapid discharge of the hot dispersion. The
agitator had three, six-bladed, impellers and was driven by a 2 HP DC
variable speed motor. A 5 gallon stainless steel discharge tank was
connected to the reactor via a 1" diameter stainless steel line. The hot
dispersion was rapidly discharged into this tank containing approximately
6.5 liters 20.degree.-23.degree. C. water at the completion of each run.
The hot dispersion was introduced below the surface of the water in the
discharge tank.
Powders produced in the examples were analyzed using laser light scattering
to measure the size distribution by volume. This technique uses the
principle of diffraction from the particles as the measurement means. A
Model 2600C Malvern Particle Size Analyzer with the proper lens
configuration for the expected particle size interfaced to a computer
which reads the diffraction pattern and performs the necessary
integrations digitally was used. For the analysis, water is charged to the
water bath and circulated through the sample measuring chamber. After
obtaining the baseline measurement, the agitator and sonic vibrator are
turned on and powder is added to the water bath until the obscuration
reading is 0.3. Mixing and circulation are controlled to obtain acceptable
dispersion without excessive foaming. A drop of liquid detergent is added
to facilitate dispersion. After eight minutes agitation, measurements are
commenced and the size distribution data are automatically tabulated. The
cumulative volume undersize and volume frequency are tabulated for 32 size
classes together with useful derived parameters. A logarithmic plot is
also produced. Duplicate runs are made for each powder sample. The
particle size reported in the examples is the number average particle size
D(v, 0.5). The range reported for particle size distributions in the
examples is for 80 percent of the volume distribution curve, i.e., from
D(v, 0.1) to D(v, 0.9). In other words, ten percent of the powder
particles are sized below the recited lower limit and 10 percent of the
powder particles are larger than the upper recited particle size limit.
This range provides a convenient means of comparing powders.
EXAMPLE I
To demonstrate the ability to produce improved polypropylene powders having
reduced fines the following example is provided.
(a) Visbreaking--A commercially available general purpose propylene
homopolymer suitable for the production of containers and housewares (MFR
5; tensile strength 4900 psi; flexural modulus 210,000 psi) was visbroken
by mixing with 3500 ppm di-(2-t-butylperoxyisopropyl)benzene. A Killion
Laboratory Extruder equipped with four heating zones (T.sub.1 350.degree.
F.; T.sub.2 375.degree. F.; T.sub.3 400.degree. F.; T.sub.4 425.degree.
F.) was employed for the visbreaking. The polypropylene contained 1500 ppm
conventional stabilizers. In a single pass through the extruder, the MFR
of the propylene homopolymer was increased from 5 to 20.
(b) Powder Formation--450 Grams of the above-prepared visbroken polymer was
charged to an autoclave with 180 grams nonionic surfactant (Pluronic.RTM.
F-98 --a block copolymer of ethylene oxide and propylene oxide of
molecular weight 13500 and containing 20% propylene oxide). Water (810
grams) was then added and the reactor was sealed and heated. When the
temperature reached 210.degree. C., agitation (3500 rpm) was commenced and
maintained for 15 minutes. The temperature ranged from 216.degree. C. to
232.degree. C. during the agitation period and a maximum pressure of 480
psi was developed in the reactor. At the completion of the stirring
interval, the dispersion was rapidly discharged into the discharge tank.
The resulting powder was recovered by vacuum filtering the slurry through
the filter cloth (96.times.74 threads; 5 harness sateen weave; standard
flow rate 15-24 cfm). The product retained on the filter was redispersed
in 2 liters water and refiltered. The wet material was air-dried to obtain
a free-flowing powder which was analyzed using the Malvern Particle Size
Analyzer. Powder characteristics are reported in Table I and FIG. 1 is the
particle size distribution plot generated for the powder showing the
percentage of particles over the entire particle size range. Microscopic
examination of the powder showed the particles to be spherically shaped.
COMPARISON A
To illustrate the improved results obtained with the process of the
invention wherein a visbroken propylene polymer is used to prepare the
powder, the following comparative experiment was carried out. For this
example a commercially available general purpose propylene homopolymer
directly synthesized to a melt flow rate of 20 and having a tensile
strength 5200 psi and flexural modulus 240,000 psi was utilized. The
general procedure described in Example I to produce the powders was
employed. The amount of polypropylene, nonionic surfactant and water were
the same as used in Example I. Agitation was maintained at 2500 rpm for 30
minutes while the temperature was maintained in the range 218.degree. C.
to 225.degree. C. The resulting powder was characterized and results are
reported in Table 1. FIG. 2 is the particle size distribution curve
generated for the powder using the Malvern Particle Size Analyzer.
COMPARISON B
To further illustrate the improvements obtained with the invention, another
comparative powder was produced utilizing the directly synthesized
propylene homopolymer used for Comparison A having a MFR of 20. For this
experiment, however, the amount of nonionic surfactant used was reduced to
135 grams. The amount of polymer and water used were the same. Agitation
time was 30 minutes at 2500 rpm and the temperature during the agitation
period ranged from 218.degree. C. to 223.degree. C. The resulting powder
was analyzed to determine average particle size and particle size
distribution. Results are provided in Table I.
EXAMPLE II
To further illustrate the invention, improved powders were produced using a
random copolymer of propylene and ethylene.
(a) Visbreaking--A commercially available general purpose propylene
copolymer resin suitable for blow molding and sheet extrusion was
employed. The polymer was a random copolymer of propylene with about 3
weight percent ethylene and had a MFR of 2, tensile strength of 4200 psi
and flexural modulus of 135,000 psi. The copolymer was visbroken by
extruding with 3500 ppm organic peroxide in a Killion extruder. The
procedure and conditions employed were the same as described in Example I.
The MFR of the visbroken material obtained after one pass through the
extruder was increased from 2 to 11.6.
(b) Powder formation--The above-prepared visbroken random copolymer (450
grams) was charged to the autoclave with 180 grams nonionic surfactant and
810 grams water and dispersed as described in Example I. To achieve
dispersion, the mixture was heated to 210.degree. C. and agitated for 15
minutes at 3500 rpm. The temperature was maintained at 215.degree. C. to
227.degree. C. over this period and the maximum pressure developed was 410
psi. The powder (melt flow rate 12-13) was recovered in the usual manner
and analyzed. Powder characteristics are reported in Table 1. FIG. 3 is
the particle size distribution plot generated for the powder using the
Malvern Particle Size Analyzer.
COMPARISON C
A directly synthesized random copolymer having a melt flow rate of 10 and
ethylene content of 3% (tensile strength 3700 psi; flexural modulus
130,000 psi) was dispersed following the general procedure of Example II.
After heating the mixture to 210.degree. C. the dispersion was agitated
for 15 minutes at 3500 rpm. The temperature was maintained at 215.degree.
C. to 228.degree. C. during the agitation and the maximum pressure was 420
psi. The comparative powder was analyzed to characterize the powder in the
usual manner and results are reported in Table 1. FIG. 4 is the plot
generated for the comparative powder showing the percentage of particles
over the entire particle size distribution range.
EXAMPLE III
To demonstrate the ability to vary the conditions in the powder forming
operation, the following experiment was conducted using the visbroken
random copolymer (MFR 11.6) produced in Example II. For this reaction, the
visbroken copolymer (450 grams) and surfactant (180 grams) were
simultaneously charged to the reactor. The water (810 grams) was then
added and the reactor was sealed. Dispersion was achieved by heating to
210.degree. C. and agitating for approximately 2 minutes at 4900-4950 rpm.
The temperature ranged from 216.degree. C. to 223.degree. C. during the
agitation period. Stirring was discontinued and the dispersion was
discharged after about 1-2 minutes. The recovered copolymer powder was
analyzed and results are reported in Table I.
TABLE 1
__________________________________________________________________________
PRODUCT I COMP. A
COMP. B
II COMP. C
III
__________________________________________________________________________
Average Particle
33.8 41.0 13.6 26.1 8.9 29.7
Size (microns)
Particles Under 10
<2.2 >26.2 >40.6 <8.0 >56.6 <4.6
Microns in Size (%)
Particle Size
18.1-60.9
3.5-108.0
2.4-56.9
11.9-49.8
2.7-17.8
15.1-51.7
Distribution
(microns)
__________________________________________________________________________
Top